The invention will be described in relation with PDP but may be applicable to other types of display as mentioned above.
As well known, a plasma display panel is constituted by two insulating plates sealed together to form a space filled with gas. Ribs are provided inside the space to form a matrix array of discharge cells which could only be “ON” or “OFF”. Also, unlike other displays such as CRT (Color ray tube) or LCD (Liquid Crystal Display) in which grey levels are expressed by analogue control of the light emission, a PDP controls the grey level by modulating the number of light pulses per frame. These light pulses are known as sustain pulses. The time-modulation will be integrated by the eye over a period corresponding to the eye time response.
In the field of video processing, an 8-bit representation of a luminance level is very common. In this case, each video level will be represented by a combination of the following 8-bits:20=1, 21=2, 22=4, 23=8, 24=16, 25=32, 26=64, 27=128
To realize such a coding scheme with the PDP technology, the frame period which has a duration function of the frequency of 16 ms for 60 Hz or 20 ms for 50 Hz, is divided in 8 sub-periods known as sub-fields SF. Each sub-field SF corresponds to one of the 8 bits as shown in FIG. 1. The duration of the light emission for the bit 21=2 is the double of that for the bit 20=1, etc. . . . With a combination of these 8 sub-periods, it is possible to build 256 different grey levels. For example, the grey level 92 will thus have the corresponding digital code word 00111010=4+8+16+64. More specifically, in known plasma display technology, each sub-field SF is a period of time comprising:                a writing/addressing period of fixed length in which the plasma cell is either brought to an excited state with a high voltage or to a neutral state with lower voltage,        a sustain period depending on the sub-field weighting. A gas discharge is made with short voltage pulses or sustain pulses with equal amplitude and equal duration, the number of pulses corresponding to the sub-field weighting,        an erasing period of fixed length in which the charge of the cells is quenched.        
In addition, a priming pulse P may be used at the beginning of the frame period. Such priming makes a pre-excitation of the plasma cell to prepare the cells for homogeneous writing of each sub-field.
So video levels are mapped to a set of sub-field codes based on the sub-field weight. Thus, luminance is generated by means of a discrete number of sustain pulses distributed by a discrete number of sub-fields. If the number of sustain pulses which have to be distributed by the sub-fields of a frame corresponds to the number of video levels, the repartition would be straightforward as in the above example wherein 255 sustain pulses have to be distributed by a sub-field group 1-2-4-8-16-32-64-128 allowing 256 different luminance values. However, if for instance 293 sustain pulses have to be distributed, the process is substantially more complicated. Sustain pulses can not be neatly divided among the sub-fields giving rounding errors. Further complication arises due to the fact that the process of writing and erasing a sub-field also generates some luminance equally added to every bit sub-field regardless of its weight. So PDP panels are slightly non-linear, i.e. 100 sustain pulses will not produce 100 times more luminance than a single sustain pulse.
As similar to CRTs, PDPs require use of a Peak White Enhancement (PWE) circuit, which controls peak white level as a function of average image power. The number of peak white sustain pulses is adapted to said average picture power and the sustain pulses can not be neatly divided amongst the sub-fields as mentioned above.
Due to problems such as rounding errors, plasma non-linearities, existence of parasitic luminance components like addressing and erasing pulses, the known solution consisting to map the number of required sustain pulses to chosen sub-field code weight structure produces clearly perceptible grey scale portrayal non-linearities.